1. Field of the Invention
This invention relates to apparatus for obtaining real time data from a sensor and more particularly to a programmable real time controller for gathering data independently of a central processing unit which programs the real time controller.
2. Description of the Prior Art
In a microwave landing system, a stationary ground based unit emits signals and operates in conjunction with an airborne receiver which receives and decodes the signals. The decoded signals are fed to instruments used to guide an aircraft during takeoff and landing by supplying angular position information with respect to a selected glide path defined by the ground based unit emitting the signals. Signals received by the airborne receiver include a highly directional microwave beam that scans a covered zone surrounding the runway at a fixed angular velocity. The airborne receiver determines the centroids of the beams as they intercept the aircraft antenna and computes the time differential between the two "scanning beams" to determine aircraft angular position. Preceding these scanning beams, omnidirectional digital data is transmitted from the ground as differentially phase encoded data on an RF carrier. The information received includes function identification and ground status information. Many different signals and formats are sent by the ground base unit transmitter and each require different data handling operations.
The airborne receiver of the microwave landing system includes a central processing unit which is responsible for intercepting the data imbedded in the signals emitted by the ground based unit in real time by way of central processing unit interrupts as well as performing the necessary processing or computation. In optimizing the design of the airborne receiver, the central processing unit was loaded with additional chores, such as more processing of signals, increased function repetition rates, increased data rates, and receiver built-in test (bit) capability. Since interrupts and interrupt handlers are time consuming, it became impractical for the central processing unit to be responsible for the gathering of data from signals emitted by the ground based unit in real time, as well as all of its other chores.
The overhead required by the central processing unit to gather data, in itself, rendered other central processing units unacceptable. Also, the use of the same central processing unit was desirable because it allowed existing software to be implemented. Increasing the central processing unit clock frequency was not desirable because of increased costs associated with higher performance parts and the criticality of keeping high frequency clocks to a minimum in radio frequency (RF) receiving systems.
It is therefore desirable to provide a real time controller which is programmable in order to operate on different signals independently of the central processing unit and to perform operations normally done by the central processing unit in real time, such as clear peak detector, generate tracking gates, store data in real time, switch between antennas, track beams in real time, and synchronize the incoming data to the central processing unit clocks.
It is further desirable to provide a real time controller which may collect data and perform other duties without the help of the central processing unit.
It is further desirable to provide a real time controller which monitors a real time clock and operates on a real time event in response to the time indicated by the real time clock.
It is further desirable to provide a real time controller which generates interrupts to a central processing unit upon completion of its duties.
It is further desirable to provide a real time controller which includes independent memory space for data storage.
It is further desirable to provide a real time controller which includes a direct memory access controller for high volume data, such as beam samples, and to position the data into the central processing unit work space, random access memory (RAM).
It is further desirable to provide a real time controller which may be programmed by a central processing unit in a simplified manner and requiring little time.
It is further desirable to provide a real time controller which includes a storage means for storing data collected from sensors and wherein a central processing unit may independently transfer the data from the storage means to itself.